1. Field of the Invention
The present invention relates to a plasma display device employing a plasma display panel (hereinafter also referred to as a plasma panel or a PDP) and an image display system using the plasma display device. In particular, the present invention is useful for providing a display device capable of improving luminous efficacy and producing a high-contrast and high-quality image.
2. Description of Prior Art
Recently, plasma display devices have been expected as promising large-size thin color display devices. More specifically, an ac surface-discharge type PDP is the most common type among PDPs put to practical use because of its simple structure and high reliability. Although the present invention will be explained mainly by using a conventional PDP of the ac surface-discharge type, the present invention is equally applicable to other types of PDPs.
FIG. 2 is an exploded perspective view illustrating a part of a structure of an example of a plasma panel. Formed on an underside of a front glass substrate (a substrate facing a viewing space explained subsequently) 21 are transparent common electrodes (hereinafter referred to as X electrodes) 22-1, 22-2 and transparent independent electrodes (hereinafter referred to as Y electrodes or scan electrodes) 23-1, 23-2. X bus electrodes 24-1, 24-2 and Y bus electrodes 25-1, 25-2 are overlaid on the X electrodes 22-1, 22-2 and the Y electrode 23-1, 23-2, respectively. Further, the X electrodes 22-1, 22-2 and the Y electrodes 23-1, 23-2, the X bus electrodes 24-1, 24-2, and the Y bus electrodes 25-1, 25-2 are covered with an dielectric 26, and then are covered with a protective film (also called a protective layer) 27 such as magnesium oxide (MgO). The X electrodes 22-1, 22-2 and the Y electrodes 23-1, 23-2, the X bus electrodes 24-1, 24-2, and the Y bus electrodes 25-1, 25-2 are collectively named a display discharge electrode or a display electrode (a display discharge electrode pair or a display electrode pair when a pair of X and Y electrodes is indicated).
In the above, the X electrodes 22-1, 22-2 and the Y electrodes 23-1, 23-2 have been explained as transparent electrodes, this is because a lighter (high-brightness) panel can be obtained, and it is needless to say that they do not always need to be transparent. Magnesium oxide (MgO) is explained as a concrete material for the protective film 27, but material for the protective film 27 is not limited to magnesium oxide. The objects of the protective film 27 are to protect the display discharge electrodes and the dielectric 26 from bombarding ions and to promote initiation and sustenance of discharge with secondary electron emission caused by incident ions. Other materials can be used which are capable of achieving the above objects. The front glass substrate 21 combined in this way with the electrodes, the dielectric, the protective films in an integral structure is called a front plate.
On the other hand, formed on an upside of a rear glass substrate 28 are electrodes (hereinafter referred to as A electrodes or address electrodes) 29 such that they intersect the X electrodes 22-1, 22-2 and the Y electrodes 23-1, 23-2 at right angles with grade separation. The A electrodes 29 are covered with a dielectric 30, and barrier ribs 31 are formed on the dielectric 30 such that they extend in parallel with the A electrodes 29. Further, phosphors 32 are coated on inner surfaces of cavities formed by wall surface of the barrier ribs 31 and the upper surfaces of the dielectric 30. The rear glass substrate 28 combined in this way with the A electrodes and the dielectric in an integral structure is called a rear plate.
A plasma panel is fabricated by bonding the front and rear plates provided with the necessary constituent elements as described above, filling a gas (a discharge gas) for creating plasma, and then sealing the panel. It is needless to say that it is necessary to bond and seal the front and rear plates to ensure the hermeticity of the sealed package containing the discharge gas.
FIG. 3 is a cross-sectional view of the PDP of FIG. 2 viewed in the direction of the arrow D1 of FIG. 2, and schematically illustrates one cell which serves as the smallest picture element with borders of the one cell roughly indicated by broken lines. Hereinafter, cells are also called discharge cells.
In FIG. 3, the A electrode 29 is disposed halfway between the two barrier ribs 31, and the gas (discharge gas) for creating the plasma is contained within a discharge space 33 surrounded by the front glass substrate 21, the rear glass substrate 28 and the barrier ribs 31.
Here, the discharge space means a space where a display discharge, an address discharge, or a preliminary discharge (also called a reset discharge) is generated in operation of the plasma panel as described later. More specifically, the discharge space is a space which is filled with the discharge gas, has applied thereacross an electric field necessary for the discharge, and has a spatial expanse required for generation of the discharge. Further, a display discharge space means a space where a display discharge occurs, more specifically, a space which is filled with the discharge gas, has applied thereacross an electric field necessary for a display discharge, and has a spatial expanse required for generation of the display discharge. The discharge space and the display discharge space mean a space included in each of the discharge cells, or a collection of the spaces included in the discharge cells.
In a color PDP, usually three kinds of phosphors for red, green and blue are coated within the cells. A trio of cells coated with the three different kinds of phosphors serve as one pixel. A space having a plurality of such cells or pixels arranged continuously and periodically is called a display space. A set is called a plasma display panel or plasma panel which includes the display space and is provided with other necessary structures such as vacuum sealing and electrode leads for external connection. Hereinafter, the plasma panel is also referred to as the PDP.
In the plasma panel, a structure integrally fabricated to seal the discharge gas therein hermetically is referred to as the basic plasma panel. In the basic plasma display panel, a surface from which visible light for display is irradiated is called a display surface, and a space into which the visible light for display is irradiated is called a viewing space.
As described above, in the basic plasma panel, there is a space containing the plural discharge cells arranged continuously, which is hereinafter referred to as a display space. A projection of the display space onto the display surface is called a display region Rp, a projection of the discharge space onto the display surface is called a discharge region, and a projection of the display discharge space onto the display surface is called a display discharge region. A region other than the display discharge region in the display region Rp is called a non-display discharge region. A projection of the discharge cell onto the display surface is called a cell region.
A direction perpendicular to the display surface is called a height direction. In a case where the discharge cells include barrier ribs as their constituent components, a direction of a line connecting centers of two adjacent ones of the discharge cells arranged with one of the barrier ribs interposed therebetween is called a width direction, and a direction perpendicular to the width direction in a plane parallel with the display surface is called a length direction.
A barrier rib width is defined as a width of the barrier rib as measured in the width direction, and an average of the barrier rib width averaged over the height direction of the barrier rib is called an average barrier rib width Wrba.
In the conventional plasma panel shown in FIG. 2, the length directions of the barrier ribs are oriented approximately in one direction, and this structure of the plasma panel is called the straight-barrier-rib structure. In another conventional plasma panel, the length directions of the barrier ribs are oriented in at least two directions, that is, DR1 and DR2, and this structure of the plasma panel is called the box-barrier-rib structure.
FIG. 4 is a cross-sectional view of the PDP of FIG. 2 viewed in the direction of the arrow D2 of FIG. 2, and schematically illustrates one cell with borders of the one cell roughly indicated by broken lines. Reference character Wgxy denotes a spacing between the display electrode pair (the X and Y electrodes), and the spacing Wgxy is called a display electrode gap. In FIG. 4, reference numeral 3 denote electrons, 4 is a positive ion, 5 is a positive wall charge, and 6 are negative wall charges.
By way of example, FIG. 4 schematically illustrates that, by applying a negative voltage to the Y electrode 23-1 and a voltage positive with respect to the Y electrode 23-1 to the A electrode 29 and the X electrode 22-1, initially a discharge is generated, and then the discharge has ceased. This has caused formation of a wall charge for assisting in initiation of a discharge between the Y electrode 23-1 and the X electrode 22-1, and this formation of the wall discharge is called address. In this state, when an appropriate voltage of the polarity opposite from the previous one is applied between the Y electrode 23-1 and the X electrode 22-1, a discharge is generated in the discharge space between the two electrodes through the dielectric 26 (and the protective film 27). After the cessation of the discharge, if the polarity of the voltage applied between the Y electrode 23-1 and the X electrode 22-1 is reversed, a new discharge is generated again. By repeating this process, discharges are generated continuously, and these discharges are called display discharges (or sustain discharges).
FIG. 5 is a block diagram illustrating an image display system including a plasma display device employing a PDP and a video signal source coupled thereto. A driving means (also called a drive circuit) receives signals representing a display scene from the video signal source, and then converts the signals into drive signals for the PDP in a procedure explained below and drives the PDP.
FIGS. 6A–6C illustrate an operation during one TV field (hereinafter also called simply one field) required for displaying one picture on the PDP shown in FIG. 2. FIG. 6A is a time chart. As shown in portion (I) of FIG. 6A, one TV field 40 is divided into sub-fields 41 to 48 each having a different number of plural light emission times. Gray scales are generated by lighting one or more selectively from among the sub-fields.
As shown in portion II of FIG. 6A, each sub-field comprises a preliminary discharge period 49, an address discharge period 50 for addressing discharge cells to be lighted, and a display period (also called a lighted display period) 51.
The preliminary discharge period 49 is a period for homogenizing conditions of all the cells (conditions for establishing their drive characteristics) and preparing to ensure stability and reliability in their subsequent operations. Usually, during the preliminary discharge period, a preliminary discharge, a reset discharge, or an overall-address discharge (a discharge for addressing the entire display region simultaneously) is performed.
FIG. 6B illustrates waveforms of voltages applied to the A electrode, the X electrode and the Y electrode during the address discharge period 50 shown in FIG. 6A. A waveform 52 represents a voltage V0 (V) applied to one of the A electrodes during the conventional address discharge period 50, a waveform 53 represents a voltage V1(V) applied to the X electrode, and waveforms 54 and 55 represent voltages V2(V) applied to ith and (i+1)th Y electrodes. When a scan pulse 56 is applied to the ith Y electrode (in FIG. 6B, the scan pulse is illustrated as ground potential, but it may be selected to be a negative voltage), an address discharge is generated in a cell located at an intersection of the ith Y electrode with the address electrode 29. Even when the scan pulse 56 is applied to the ith Y electrode, if the A electrode 29 is at ground potential, the address discharge is not generated.
In this way, each of the Y electrodes is supplied with the scan pulse once during the address discharge period 50, and the A electrodes 29 are supplied with the voltage V0 or ground potential in synchronism with the scan pulse according to whether they are to be lighted or not to be lighted, respectively. In the discharge cells where the address discharges have been generated, electric charges are formed by the discharges on the surfaces of the dielectric and the protective films covering the Y electrodes. ON and OFF of the display discharge described subsequently are controlled by the assistance of an electric field generated by the above-mentioned electric charge. That is to say, the cells which have generated the address discharge serve as lighted cells, and the remainder of the cells serve as non-lighted cells.
On the other hand, there is another driving method in which the cells which have generated the address discharge serve as non-lighted cells (in which a wall charge generated by the above-explained overall-address discharge is eliminated by the address discharge), and in which the remainder of the cells serve as lighted cells.
FIG. 6C illustrates display discharge pulses applied between the X and Y electrodes which serve as display electrodes (also called display discharge electrodes) all at the same time during the display period 51 shown in FIG. 6A. The X and Y electrodes are supplied with the voltage waveforms 58 and 59, respectively.
The pulses of the magnitude V3 (V) and the same polarity are applied alternately to the X electrodes and the Y electrodes, and as a result reversal of the polarity of the voltage between the X and Y electrodes is repeated. The discharge occurring in the discharge gas between the X and Y electrodes during this period is called the display discharge. Here, display discharges occur in pulses, and their polarities are alternated.
A display electrode-to-electrode voltage Vse(t) externally applied in a cell during the display period is expressed byVse(t)=Vy(t)−Vx(t)  (1)where Vx(t) and Vy(t) are voltage applied to the X and Y electrodes, respectively, during the display period, and t represents time.
A maximum applied display-discharge voltage Vsemax is defined as the maximum of the absolute value |Vset(t)| of the display electrode-to-electrode voltage Vse(t) during a time when the display discharge pulses are applied. In FIG. 6C, Vsemax is V3 (V). However, in a case where the waveshape of the voltage actually applied to the display electrodes is distorted by capacitances, inductances and resistances and others included in circuits on route from the power supply to the plasma panel, and consequently, is not rectangular unlike in the case of FIG. 6C, V3 represents the display electrode voltage averaged over a time when the display discharge pulses are applied, and therefore Vsemax has a magnitude somewhat different from that of V3.
Usually the means for generating the display discharge pulses is provided in the drive means shown in FIG. 5. FIG. 7 illustrates its outline. The means for generating the display discharge pulses includes as its constituent elements dc voltage supplying means, that is, display-discharge dc power supplies, and switch circuits (circuits X, Y in FIG. 7) provided between the display-discharge dc power supplies and the display electrodes. The display-discharge dc power supplies may be formed of mere capacitors, or may be formed of mere grounding electrodes (grounding interconnection lines). The switch circuits serve to select voltages from among output voltages of the display-discharge dc power supplies including ground potential and apply the selected voltages to the display electrodes. A display-discharge dc power supply voltage Vsdc is defined as the maximum of the absolute value of a difference between two output voltages from the two display-discharge dc power supplies, respectively. The display-discharge dc power supply voltage Vsdc is approximately equal in magnitude to V3. However, in a case where the waveshape of the voltage actually applied to the display electrodes is distorted by capacitances, inductances and resistances and others included in circuits on route from the power supply to the plasma panel, and consequently, is not rectangular unlike in the case of FIG. 6C, Vsdc has a magnitude somewhat different from that of V3.
In the above explanation, the display discharge has been explained in connection with a driving system in which the address discharge periods and the display periods are separated from each other, that is, the Address and Display Periods Separated Driving System, but the essence of the display discharge lies in intentional generation of light emission necessary for display, and therefore it is needless to say that such a discharge is recognized as the display discharge in other driving systems also.
For example, in the above-explained driving system (the Address and Display Periods Separated Driving System), the address discharge periods and the light-emission display periods are provided for the entire display region simultaneously, respectively. However, there is another driving system in which, while the address discharge periods are provided to some of the scanning electrodes (the Y electrodes), the light-emission display periods are provided to others of the scanning electrodes (the Y electrodes), and vice versa, and this driving system is called the Simultaneous Address and Display Driving System.
In the above-explained conventional techniques, the so-called progressive scanning drive system is employed, and all the discharge cells in the display region are used for displaying an image during each field period. On the other hand, the so-called interlaced scanning driving system can also be used. In the interlaced scanning driving system, the discharge cells of the plasma panel are divided into two kinds (group A and group B, for example), an image display is performed by alternately using the discharge cells of each of the group A and the group B on successive fields. For example, successive fields are divided into odd-numbered fields and even-numbered fields, and an image display is performed by using the discharge cells of the group A on the odd-numbered fields and using the discharge cells of the group B on the even-numbered fields. Further, in a third driving system, the same scanning electrodes (Y electrodes) may be used both for driving the odd-numbered fields and for driving the even-numbered fields. The plasma display device employing the plasma panel to which the interlaced scanning driving system or the above-described third driving system is applied is called the ALIS (Alternate Lighting of Surfaces) type plasma display device. The details of the ALIS type plasma display device have been reported in Kanazawa, Y., T. Ueda, S. Kuroki, K. Kariya and T. Hirose: “High-Resolution Interlaced Addressing for Plasma Displays,” 1999 SID International Symposium Digest of Technical Papers, Volume XXX, 14.1, pp. 154–157 (1999).